Many electrochemical cells, such as capacitors, batteries or rechargeable batteries, have a housing in the form of a cup (i.e., a cup-shaped, or substantially cylindrical, housing) that accommodates (i.e., holds) an electrode stack. The electrode stack generally comprises flat positive and negative electrodes that can be separated from one another via, e.g., a separator layer. The electrodes, in this example, make contact with an electrolyte. In the case of electrolytic capacitors, such as aluminium electrolytic capacitors, the electrodes frequently comprise an aluminium cathode film and an anode film comprised of aluminium with a dielectric oxide layer. A spacer is located between the films. The spacer may be a single layer or a multiple layer comprised of paper that is impregnated with an electrolyte solution. The arrangement is normally implemented as a winding that is applied around a mandrel and that is introduced into the cup-shaped housing. A cover, on which electrical connections are arranged, is frequently used to close the top of the cup-shaped housing. In this case, and particularly in the case of electrolytic capacitors, the connections can be electrically conductive connections to the capacitor winding.
Electrochemical cells, such as those described above, are frequently used in automotive applications, such as automobiles. There, these cells are subject to very severe mechanical vibrations. If the vibration loads are severe enough, the electrode stack may move relative to the cup-shaped housing. As a result, it is possible for the electrodes of the electrode stack to be damaged, or for the electrical connections between the electrode stack and the electrical connections which are fitted on the outside of the housing to become loose or to be damaged.
An electrolytic capacitor having a high vibration load capacity is known from Laid-Open Specification DE 199 29 598 A1. This electrolytic capacitor has connecting strips between the capacitor winding and the two electrical connections. The connecting strips absorb the majority of the forces which place a load on the capacitor winding when the entire capacitor is subject to vibration. In addition, the capacitor winding may also be fixed in the housing by so-called center beads with a cross section, which tapers into the interior of the housing being fitted. The center beads make contact with the capacitor winding. However, these center beads have a linear contact area with the capacitor winding. Beads such as these do not adequately fix the capacitor winding to the cup-shaped housing, necessitating use of the connecting strips mentioned above.
The object of the present invention is thus to provide a vibration-resistant electrochemical cell, which can be produced particularly easily, together with a method for producing the electrochemical cell, which avoids the disadvantages mentioned above.
According to the invention, this object is achieved by an electrochemical cell according to claim 1. Further advantageous embodiments of the cell and production method are the subject matter of further claims.
An electrochemical cell according to the invention includes a cup-shaped housing that accommodates an electrode stack. At least one indentation is provided in the center housing. The indentation fixes (i.e., holds) the electrode stack in the housing. A two-dimensional, flat contact area is, in this case, formed between the indentation and the electrode stack.
Since, in the invention, there is a flat contact area between the indentation and the electrode stack, a particularly large contact area can be produced, which can fix the electrode stack in the housing particularly well and reliably. Conventional indentations in housings of electrochemical cells have a cross section, such as a round cross section, which tapers into the interior of the housing, and which allows only a linear contact area between the indentation and the electrode stack. Thus, housings with conventional indentations are not able to fix an electrode stack reliably when subject to high vibration loads.
A further advantage of an electrochemical cell according to the invention is that the large contact area between the housing and the electrode stack allows better thermal contact to be established. An alternating current load frequently results in heat in the electrode stack, particularly in the capacitor winding of capacitors. Such heat can be dissipated better to the housing and from there to the environment via the particularly large contact area with the housing.
Advantageously, the at least one indentation is formed in side walls of the cup-shaped housing. Also advantageously, there are areas at the edge of the indentation, which are indented more deeply into the interior of the housing than other areas of the indentation. This enables pressure forces to be distributed more uniformly over the electrode stack and, as a result, ensures that the electrode stack is not pinched severely.
It is advantageous for at least three more deeply indented areas to be provided in the indentation. The three more deeply indented areas may represent, e.g., the boundary points in a particularly simple manner for an area in which a flat indentation according to the invention can be produced in a housing.
In order to ensure that the electrode stack is fixed particularly well in the housing, the indentation in the housing advantageously extends over the majority of the height of the electrode stack.
It is also possible that two or more indentations are provided in the housing of the electrochemical cell, which are formed either in the lower area or in the upper area of the electrode stack in the housing. The use of indentations for alternate fixing in the upper and lower areas of the electrode stack likewise makes it possible to ensure that this electrode stack is fixed particularly reliably.
The electrode stack may, in this case, have at least two electrode layers, which are separated from one another by a separator layer. If the electrochemical cell is in the form of an aluminium electrolytic capacitor, then the electrode stack may be a capacitor winding which surrounds two aluminium films as electrodes, which are separated from one another by a separator and make contact with an electrolyte. The anode film may, in this case, have a dielectric oxide, such as aluminium oxide. The separator layer may be comprised of, e.g., one or more layers of paper, which are impregnated with an electrolyte.
One method for producing an electrochemical cell according to the invention comprises the following steps: In a first method step (A), an electrode stack is inserted into a cup-shaped housing. Then, in a method step (B), an indentation with a flat contact area with the electrode stack is formed via a die having a die head. There are at least three contact points provided on the die head which make point contacts with the housing, and in the process fix (i.e., hold) the electrode stack to the housing.
At least three contact points are necessary in order to define the corner points of a flat area in which an indentation is formed in the housing.
A die whose die head is formed such that it forms only point and linear contact areas with the housing during production of the indentation is advantageously used in method step (B). This type of die allows the indentations to be produced particularly well with a two-dimensional, flat contact area with the electrode stack in cup-shaped housings. At the start of the stamping process, the die head makes contact with the housing only via the at least three contact points. As the die head penetrates further into the housing, this results in the housing making contact with edges of the die head that are located between the contact points. The contact areas between the edges of the die head and the housing are linear in this case. This special shape of the die head makes it possible to prevent the conventional outward bulges with a rounded cross section particularly well, which can be produced when using conventional dies (see, for example, FIG. 2).
A die is used in method step (B) in which the surface of the die head has a concave curvature, and in which the edges of the die head located between the contact points are curved such that their distance from the housing increases as the distance from the contact points increases. The edges, in each case, have a distance from the housing that is greater at approximately a center between the contact points. A die head which is shaped in this manner enables point and line contact areas with the cup-shaped housing to be formed particularly well (see, for example, FIGS. 3 and 4B). As already mentioned above, at the start of the stamping process, only point contact areas exist between the contact points of the die head and the cup-shaped housing. As the die head penetrates further into the housing during the stamping process, curved linear contact areas are also formed via the edges between the contact points. Advantageously, this allows indentations with flat contact areas to be formed adjacent the electrode stack at the end of the stamping process, as a function of the distance between the contact points and the height and the thickness of the cup-shaped housing (see, for example, FIGS. 6A to 6D).
Advantageously, as a result of the linear contact areas between the edges of the die head and the housing, elongated indentations can be produced using the dies noted above. In this case, long sides of the indentations are produced particularly easily and reliably via the linear contact areas (see, for example, FIGS. 6A to 6D).
Indentations with smaller areas can also be produced via dies which form only point contact areas with the housing throughout the entire stamping process (see, for example, FIGS. 4A and 5A b to 5E).
The dies used may be, for example, dies with a polygonal cross section, with contact points that protrude (as projections) from the surface of the die head representing the corners of the polygon. In this case, the expression xe2x80x9cpolygonal cross sectionxe2x80x9d means the projection of the plan view of the die head onto a plane.
The polygonal cross section of the die may, for example, be rectangular, in which case there is then a contact point at each of the four corners of the die head.
In method step (B), it is also possible for an indentation to be formed such that there are areas at the edge of the indentation which are indented more deeply than the rest of the indentation. These areas of deeper indentation, in this case, represent those areas of the housing which make contact with the contact points on the die head at the start of the stamping process (production of the indentation). Indentations such as these frequently do not have a surface with a planar cross section, but have a surface which is curved in a slightly convex shape outwards in areas which are not associated with the edges (see, for example, FIG. 9). Advantageously, indentations such as these can distribute pressure particularly well over the electrode stack, while at the same time fixing (i.e., holding) the electrode stack particularly reliably in the housing.
The material of the housing is advantageously a ductile material, in which the desired indentations can be produced particularly easily by means of the dies mentioned above. In this case, it is particularly advantageous to use metals, such as aluminium, as the material used to produce the cup-shaped housing.
The electrochemical cell according to the invention, and a method for producing the electromechanical cell, will be explained in more detail in the following text with reference to exemplary embodiments and figures.
Other features and advantages will be apparent from the description, the drawings, and the claims.